mybatis的association用法，表示has-one
前提：Student(id,name,supervisor_id), Teacher(id,name)，其中student.supervisor_id关联teacher.id，表示一个学生只有一个导师，一对一关系。
重要说明：column属性对应的SQL语句中，定义的别名（如果定义的话）
写法一：嵌套的select语句
<resultMap>
<association property="supervisor" column="supervisor_id"
select="findTeacherById" />
</resultMap>
<select id="findTeacherById" parameterType="int" resultMap="TeacherResultMap" >
select *
from t_Teacher
where id=#{id}
</select>
<select id="findStudentById" parameterType="int" resultMap="StudentResultMap" >
select *
from t_Student
where id=#{id}
</select>
写法二： 嵌套的resultMap，直接在association中映射t_teacher表
<resultMap>
<association property="supervisor" javaType="Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
</resultMap>
<select id="findSuperVisorByStudent" parameterType="int" resultMap="StudentResultMap">
select ts.id as s_id,
ts.name as s_name,
ts.supervisor_id as s_supervisor_id,
tt.id as t_id,
tt.name as t_name
from t_student ts left outer join t_teacher tt
on ts.supervisor_id = tt.id
where ts.id = #{id}
</select>
写法三： 嵌套的resultMap，在外部映射t_teacher表
<resultMap>
<association property="supervisor" resultMap="supervisorResultMap" />
</resultMap>
<resultMap type="Teacher" id="supervisorResultMap">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</resultMap>
<select id="findSuperVisorByStudent" parameterType="int" resultMap="StudentResultMap">
select ts.id as s_id,
ts.name as s_name,
ts.supervisor_id as s_supervisor_id,
tt.id as t_id,
tt.name as t_name
from t_student ts left outer join t_teacher tt
on ts.supervisor_id = tt.id
where ts.id = #{id}
</select>
